Single track railroad signal system using coded track circuits



C. B. SHIELDS SINGLE TRACK RAILROAD SIGNAL SYSTEM USING CODED TRACKCIRCUITS Dec. 27, 1955 6 Sheets-Sheet 1 Filed Aug. 22, 1951 1 mggww NR fvfim Eww N am {N H W k n 5% SN gunk mg 1% ham INVENTOR. Charles B.Shields HIS A T TORNE' Y C. B. SHIELDS SINGLE TRACK RAILROAD- SIGNALSYSTEM Dec. 27, 1955 USING CODED TRACK CIRCUITS 6 Sheets-Sheet 2 1 FiledAug. 22, 1951 HH| H| H H HHH 1 HH HHH H WI HH HI HHHI I I HH H l l l R wif W "1 TN w? n w m I H u u n u I 5 m s k T a z a w m I Y B I HI: n u.....|...L rl l P l l l HHHHH h HTNQ n J A E +W a mm Efiwmx EQWR m ENRNSR wa Mm Em \fi CHE 5% {i Q 7 c. B. SHIELDS 2,728,851 SINGLE TRACKRAILROAD SIGNAL SYSTEM USING CODED TRACK CIRCUITS 6 Sheets-Sheet 3 1 w.l I b flu mm Y mh E Q m W W m I I l I I I I nlnh 1B 0 5 m i M I. QmMNQQQZ ISSNQ IWQ I r 1AA n n a S w n fis w m I. Q N I I afi 5R5 I" H L EB -L FTHHMHHHH Ill l I I I I .Il K3 m fi mm Jk mkmgw 55% m F mrllli I w?g If Ra ga RY KEN Q R Dec. 27, 1955 Filed Aug. 22, 1951 C. B. SHIELDSSINGLE TRACK RAILROAD SIGNAL SYSTEM Dec. 27, 1955 USING CODED TRACKCIRCUITS 6 Sheets-Sheet 4 Filed Aug. 22, 1951 u. mm

NY... QMH

RM 5 mm H E MR m n: b a S T 0 T F fl k A E m w a in U H L M E Dec. 27,1955 c SHIELDS 2,728,851

SINGLE TRACK RAILROAD SIGNAL SYSTEM USING CODED TRACK CIRCUITS 6Sheets-Sheet 6 Filed Aug. 22 1951 Normal Code ".5SPL'.LI.5SQC. Generatedby 5067az2d 5007p J L Reverse Cocle Generated by so and azure]: al'jm lL L I 75 Uocle 120 Code W Scramble oi 50 Normal Gode and 7511mm 15o LeFtCode.

Fig 2 dud. WT

HIS ATTORNEY United States Patent SINGLE TRACK RAILROAD SIGNAL SYSTEMUSING CODED TRACK CIRCUIT S Charles B. Shields, Penn Township, AlleghenyCounty, Pa., assignor to Westinghouse Air Brake Company, Wih merding,Pa., a corporation of Pennsylvania Application August 22, 1951, SeriaiNo. 243,015

11 Claims. ((31. 246-3) My invention relates to a single track railroadsignal system using coded track circuits. More specifically my inventionrelates to a two direction signal system of the above character which issubject to remote control.

An object of my invention is the provision of an improved normallyenergized code signal system for a single track railroad.

A further object of my invention is the provision of a single track twodirection signal system incorporating normally energized coded trackcircuits which permit track circuits of the maximum permissible length.

Another feature of my invention is the provision of a signal system ofthe type here involved for a stretch of railroad commonly called a blockincorporating novel means Wherewith a block indicator is controlled whena train occupies the block or an obstruction or other fault occurs inthe track anywhere within the block limits.

Again, a feature of my invention is the provision of a single track twodirection signal system for a stretch of railroad incorporating improvedmeans for concentrating at the entering end of the stretch all thecontrols for establishing the direction of traffic.

Still another feature of my invention is the provision of a single tracktwo direction signal system for a stretch of railway incorporating anapproach signal having at least three aspects in the approach to eachleaving end of the stretch and arranged so that a following move can bemade when the track circuit beyond an approach signal is occupied by atrain moving in the same direction.

Another feature of my invention is the provision of a novel single tracktwo direction signal system for a block which requires but a singleremote control to be sent to the entrance end of the block to clear thesignal to permit a train to enter the block.

Another feature of my invention is the provision of a signal system ofthe type here contemplated incorporating novel means which enable thedirection of traflic in the block between'two passing sidings to beestablished under emergency conditions, such as a failure of a linecircuit of an associated remote control system.

Another feature of the invention is the provision of a railroad signalsystem of the type described having means for approach controlling thesignals.

A further feature of my invention is the provision of a single track:two direction signal system for a stretch of track incorporating novelmeans for obtaining the unlock of an electricallyloeked track switch ata track located intermediate the two ends of the stretch.

Other features, objects and advantages of my invention will appear asthe specification progresses.

The foregoing objects, features and advantages of my invention I obtainby providing a signal system based on the principle that when a block orstretch of a single track railroad is normal, that is, not occupied by atrain, current of a low code rate is sent from, say, the left-hand endof the stretch to the right-hand end, the current being cascaded throughthe track circuits the whole way through Z,7Z8,ti5l Patented Dec. 27,1955 the block. The code rate of this track circuit current is low andmuch lower than the code rates used to reflect different trafficconditions. At the right-hand end of the block, a reverse code of thelow code rate is generated and is cascaded through the track circuits tothe left-hand end of the block. Track occupancy detecting relays, one ateach end of the block, are normally energized in response to thesepulses of the low code rate and are used to provide an indication thatthe block is unoccupied. If a control for clearing a signal governingthe entrance of a train into this block is sent, say, to the left-handend, then the normal low code rate current is cut off and the detectorrelay at the right-hand end of the block is released and cuts off thereverse low code rate current. This operation allows current of either aor code rate to be sent through the track circuits from the right to theleft, the code rates of 75 and 120 being used to reflect approach andclear traffic conditions, respectively. It is to be understood that thecode rates of 75 and 120 are used by way of illustration and other coderates can be used.

The. sending of either the 75 or 120 code current throughv the trackcircuits causes the corresponding aspect of the signal at the left-handend of the block to be displayed. Similarly, if a control is sent to theright-hand end of the block the low reverse code for the track circuitsis cut off and current pulses of either the 75 or the 120 code aresupplied at the left-hand end and cascaded through the track circuits toclear the entering signal at the right-hand end of the block.

Slow acting detector relays are provided for each junction of the trackcircuits, one for each direction of traflic, and these relays areretained energized by the low code track circuit current and thus anobstruction or fault occuring in any track circuit Will be at oncereflected by the release of a corresponding detector relay for thecorresponding section. No intermediate Wayside signals are providedexcept an approach signal for each end and hence the direction oftrafiic can be established by a single control sent to the end of theblock at which the train is to enter. These approach signals provide theusual three aspects, clear, approach, and stop and proceed, and afollowing move is permissible when the track section in advance of anapproach signal is occupied by a train moving in the same direction.

To enable a track switch for a spur track at some intermediate point inthe block to be electrically locked, I provide a short detector trackcircuit adjacent this switch and means for repeating the code of thetrack circuits around this detector track circuit. This detector trackcircuit is used to provide the usual control of an electric lock for thetrack switch.

I shall describe a preferred form of apparatus embodying my inventionand shall then point out the novel features thereof in claims.

Figs. 1a to la, inclusive, of the accompanying drawings when placed endto end with Fig. In at the left, are a schematic view showing one formof apparatus embodying my invention when applied to a single trackrailroad extending between two passing sidings and having anelectrically locked track switch at an intermediate point. Fig. 2 is adiagrammatic view showing a series of graphs which illustrate the codesof the track circuit current under different conditions of the system.

Referring to the drawings, there is shown a stretch of single trackrailroad between passing sidings 6P8 and SP8, the two rails beingidentified by the reference characters 1a and 1b. This stretch of trackis commonly referred to as a block and it will be so designated in thepresent application, and traffic moving through this block in eitherdirection is under the control of wayside signals, leaving or blocksignals 6RA and 6RB, entering signals 6LA and 6L8 and an approach signal7L being provided for location 6 at the siding 6P3. Similarly, location8 at the siding SP8 is provided with block signals 8LA and 8L3. enteringsignals SRA and SEE and anapproach signal 7R. These wayside signals maybe of any standard type and are illustrated as being color lightsignals.

This stretch of railway track is formed by the usual insulated railjoints into a series of consecutive track sections which comprisesections 5T, 6RT, 7LT, AT, 7RT, 8LT and 9T. The track sections 5T, ATand 9T are relatively short and the remaining sections are relativelylong.

Each of the track sections is provided with a track circuit. The trackcircuits for the short detector sections 5T, AT and 9T are of the steadyenergized type and include a battery connected across the rails at oneend of the section and a direct current track relay connected across therails at the other end of the section as will be apparent by aninspection of the drawings. The track circuits for the'remaining tracksections 6RT, 7LT, 7RT and 8LT are of the direct current coded type.Each of these track circuits includes a battery, a polar bias codefollowing relay and a code transmitting relay at each end of thesection. These polar bias relays are of the well known type and it issuflicient for the present application to point out that these relaysare provided with an armature that is biased to a released positionclosing a back contact. The armature is held in this released positionwhen current flows in the winding of the relay in a reverse direction,and the armature is operated to a picked up position closing a frontcontact when current flows in the relay winding in a normal direction.In the drawings the direction of current flowing in the relay windingrequired to pick up the relay is indicated by an arrow placed on therelay winding. Each code transmitting relay is provided with a codingcontact operable to a first and a second position.

It is-to be pointed out that in addition to the track batteriesprovided, a source of current is also provided at each of the differentlocations and which source of current is preferably a direct currentsource, such as a battery, not shown, the positive and negativeterminals of this source of power being indicated on the drawings by thereference characters B and N.

Referring to the coded track circuits, the circuits are normallysupplied with current of low code rate and which code is cascaded fromthe left-hand end of the block to the right-hand end where a low ratereverse code is generated and cascaded back to the left-hand end of theblock during the off code period of the normal code. To effect this lowcode track circuit current, a code transmitter or coder 30CT is providedat location 6. The code transmitter 30CT is constantly energized andoperates to alternately close front and back contacts 10 at a low coderate which I shall assume as being 30 times per minute or one code cycleeach two seconds. It is to be understood that other low codes of thisorder can be used. The coder 30CT completes at its back contact 10 asimple circuit for a slow releasing code repeater SUCTP, this coderepeater being of the relay type which is made slow releasing by any ofthe well known arrangements. The coder 30CT and the repeater 30CTP bytheir action control a circuit for a code transmitting or coding relay6RCTPR, the circuit including terminal B of the power source, frontcontact 10 of coder 3lCT, front contact 14 of repeater 30CTP, backcontact 15 of a remote controlled relay 6RHSR, front contact 16 of ablock detector relay 6RTFPR, front contact 17 of track relay STR, frontcontact 18 of an approach locking relay 6RAS, winding of relay 6RCTPRand terminal N of the power source, these several control relays herementioned being more fully referred to later. The code transmittingrelay 6RCTPR when operated to close its front contact 19 connects atrack battery 6131 and a polar bias code following relay 6TR in seriesacross the track rails of the associated track section 6RT, and whenoperated to close its back contact 19 disconnects the track battery 681and connects the relay 6TR directly across the rails of the associatedsection. For reasons to appear hereinafter, a polar bias approach relay6LAR is connected in multiple with the track relay 6TR over frontcontact 67 of the coding relay 6RCTPR. The battery 681 is poled for thepulse of current supplied to the rails to be of what I shall call normalpolarity and it flows in the relay 6TR in the direction opposite to thatindicated by the arrow and relay 6TR does not respond thereto. Theapproach relay 6LAR is connected for current of normal polarity suppliedfrom battery 6131 to flow therein in the direction to pick up this relaybut the relay is adjusted so that it does not pick up at the magnitudeof current ordinarily supplied and the relay is icked up only when themagnitude of the current is increased in a manner to be described later.

As stated hereinbefore, the code rate of the normal track circuitcurrent is 30 pulses per minute or one code pulse every two seconds. Thearrangement is such that the on code period of the pulse supplied to thesection 6RT is substantially one-half second and the off code period isapproximately one and one-half seconds, this normal code beingillustrated at the top graph of Fig. 2.

This normal code pulse of track circuit current fiows in the rails ofsection 6RT to the right-hand end of the section and flows in aconnection including the winding of a polar bias code following relay6LTR and back contact 21 of a polar bias code following relay 7RTR forthe section next to the right. This pulse of track circuit current ofnormal polarity flows in the winding of relay 6LTR in the directionindicated by the arrow and that relay is picked up closing front contact22 to complete a simple circuit for a slow release detector relay6LTFPR. This relay is provided with slow releasing characteristics byshort circuiting paths for its winding, and one of which paths includesa resistor 23, a capacitor 24 and back contact 13 of a relay '7LSR to bereferred to later. This path is proportioned so that the relay isretained picked up as long as code pulses of the low code rate arerecurrently supplied thereto.

For reasons which will be apparent when the operation of the apparatusis described, the slow release period for relay LTFPR is varied bydisconnecting the path including resistor 23 and capacitor 24 andsubstituting a path which includes a rectifier 20. Similarly, otherrelays of the apparatus are provided with different slow releaseperiods.

The operation of contact member 68 of the code following relay 6LTRcauses current to be alternately Supplied to two portions of the windingof a decoding transformer 7LD in a manner well understood and to whichtransformer two decoding units 120DU and DU are connected. The decodingunits DU and 75DU are shown in a conventional manner since they are of astandard form and their specific structure forms no part of my presentinvention. It is sufficient for the present application to point outthat the transformer 7LD and the associated decoding units 75DU and120DU are tuned such that a relay 7LDR connected to the decoding unit12(lDU is effectively energized only when the pulses of current suppliedto the transformer 7LD are of a frequency corresponding to operation ofthe relay 6LTR at the 120 code rate.

Similarly, a relay 7LH connected to the output of the decoding unit 75DUis effectively energized and picked up only when the code pulsessupplied to the transformer 7LD are of the 75 code rate. Consequently,operation of the relay 6LTR in response to the low code rate of 30pulses per minute is ineffective to energize either relay '7LH or TLDR.it is to be observed that other decoding transformers and associateddecoding units which are provided at different places in the apparatusof my invention operate in the manner just described for the unitscontrolled by the code following relay 6LTR.

The closing of front contact of the code following relay 6LTR completesa circuit for a code transmitting or coding relay 7RCTPR for the nextsection 7LT, this circuit including the power terminal B, front contact25 of relay 6LTR, back contacts 26 and 27 of signal control relays 7LSRand 7LHR, respectively, to be referred to later, winding of relay 7RCTPRand terminal N of the power source. Hence the coding relay 7RCTPR isoperated in step with the code operation of the code following relayoLTR, and with this coding relay thus operated there is supplied codedcurrent from a battery 781 to the rails of the section 7LT due to theconnection across the rails from battery 731 over front contact 28 ofthe relay 7RCTPR and winding of the polar bias code following relay7RTR. Again, the battery 781 is poled for the pulse supplied to thesection 7LT to be of normal polarity and the connection for the codefollowing relay 'TRTR is such that it does not respond to the codepulses of current of normal polarity.

This pulse of track circuit current flows in the rails of section 7LTand is received at the right-hand end of the section to flow in aconnection including code following relay 7LTR and back contact 31 of acode transmitting or coding relay 'iLCTPR associated with the section7LT. This normal code pulse of track circuit current for section 7LTwill be substantially that shown in the top graph of Fig. 2. That is,this pulse of track circuit current for the section 7LT is substantiallyin step with the pulse of track circuit current supplied to the section6RT.

The relay 7LTR is connected so that it responds to the normal code pulseof track circuit current and the relay operates to control a codingrelay 7RCTP and repeat the code in the track section '7RT to the rightof the detector section AT providing the track section AT is unoccupied.To this end a circuit is provided that extends from terminal B throughfront contact 32 of track relay ATR for the section AT, front contact 33of the code following relay 7LTR, front contact 34 of a relay NWR to bereferred to later, and winding of coding relay 7RCTP to terminal N. Thecoding relay 7RCTP on picking up to close front contact 35, completes aconnection for supplying a code pulse from battery 7B2 to the rails ofsection 7RT, the connection being completed 4'.

through the winding of polar bias code following relay 7RRTR. Thebattery 732 is poled for the current supplied to the section 712T to beof normal polarity and the connections of the relay 'JRRTR are such.that it does not respond to this pulse of normal polarity.

The pulse flows in the rails and is received at the righthand end of thesection 7RT to flow in a connection including the winding of the polarbias code following relay 7RLTR and back contact 36 of a codetransmitting or coding relay '7LCTP associated with the section 8LT, thecurrent flowing in the relay 7RLTR in the direction to pick up thatrelay. With relay 7RLTR picked up closing front contact 37, a simplecircuit is completed for energizing a slow releasing detector relay7LTFPR and that relay is picked up and then retained energized as longas code pulses of this low code rate are received.

When the code following relay 7RLTR is picked up closing its frontcontact 38, it completes a :onnection by which current is supplied frombattery 8B1 to the rails of section 8LT, the connection including thewinding of a polar bias code following relay SRTR. Again, the battery8B1 is poled to supply pulses of normal polarity and the connections ofrelay 8RTR are such that it does not respond to code pulses of energy ofnormal polarity.

At the right-hand end of the section 8LT, the normal code pulse isreceived by a connection including the winding of a polar. bias codefollowing relay 8LTR and back contact 39 of a codetransmitting or codingrelay SLCTPR, the current flowing in the winding of the relay currentpulses to be supplied to the primary winding 42' of a decodingtransformer 8D, to which winding there is connected decoding units DUand DU, but neither of these units responds to the low code rate of thenormal track circuit pulses because they are tuned to resonance atfrequencies corresponding to current pulses of the 75 and 120 coderates, respectively.

The decoding transformer 8D is also used as a code pulse generatingmeans and is provided with a secondary Winding 43, in which voltages areinduced in response to the current pulses supplied to winding 42 due tothe code operation of the relay SLTPR, the induced voltage being of,say, normal polarity when the relay SLTPR is picked up and being ofreverse polarity when the relay SLTPR is released. The winding 43 isincluded in a circuit extending from the upper terminal of the winding43' through back contact 44 of a remote controlled relay SLHSR, frontcontact 45 of block detector relay SLTFPR, front contact 46 of anapproach locking relay SLAS, front contact 47 of track relay 9TR for thesection 9T, winding of coding relay SLCTPR and thence to the lowerterminal of the winding 43 through the common connection N. The codingrelay SLCTPR is of the polar bias type and it is connected so that itresponds to the polarity of the voltage induced in the winding 43 onlywhen the code repeater relay 8LTPR is released.

With the coding relay SLCTPR picked up closing its front contact 39, aconnection is completed by which a pulse of current issupplied from thebattery 8B2 to the rails of section 8LT, the connection including thewinding of polar bias code following relay 8LTR, which, as pointed outabove, is connected to respond to the normal code pulse of track circuitcurrent.

The battery 832 is poled for the current supplied therefrom to thesection 8LT in response to operation of relay SLCTPR by the voltageinduced in winding 43, to be of the polarity reverse to the normalpolarity of the normal code pulses cascaded through the track sectionsin the manner just described. Thus the relay 8LTR does not pick up inresponse to the reverse pulse supplied from battery 8B2.

A polar bias approach relay SRAR and front contact 7!) of the codingrelay SLCTPR from a path in multiple with the code following relay 8LTRand the current pulse of reverse polarity supplied from battery 8B2 tothe rails in the manner just described flows in the approach relay SRARin a direction to pick up that relay but the relay is proportioned sothat the magnitude of the portion of the pulse of current flowingtherein is insufficient to pick up the relay except under conditions tobe explained hereinafter.

Furthermore, the coding relay SLCTPR is operated by the induced voltagein the winding 43 in such a manner and for a period such that thereverse pulse of track circuit current supplied when the relay is pickedup closing front contact 39 is of substantially one-half second orone-fourth of the code cycle of the low code rate of the track circuitcurrent. This reverse code generated at the location 8 is illustrated inthe second graph from the top of Fig. 2.

This reverse code pulse flows in the rails of the section 8LT and at theleft-hand end of, the section it flows in a connection which includesthe winding of the polar bias code following relay 8RTR and back contact38 of relay 7RLTR, the current flowing in the relay SRTR in thedirection proper to pick up that relay. The picking up of the relay SRTRclosing front contact 48 completes a simple circuit by which current issupplied to a slow releasing detector relay SRTFPR and that relay ispicked up and retained picked up as long as code pulses of the low coderate are received. The relay 8RTFPR is provided with different slowrelease periods by paths connected across its winding in a mannersimilar to relay 6LTFPR at the location of signal 7L.

Operation of the contact member 71 of the relay 8RTR completes aconnection by which current is supplied to a decoding transformer 7RD towhich 75DU and 120DU decoding units are connected by these units do notrespond to operation of the relay SRTR by the low code rate of thecurrent pulses due to the tuning of the units.

The closing of from contact 51 of the code following relay SRTRcompletes a circuit for energizing coding relay 7LCTP, the circuitextending from terminal B through front contact 51 of relay 3RTR, backcontact 52 of signal control relay 7RSR, back contact 53 of a signalcontrol relay 7RHR, and winding of coding relay 7LCTP to terminal N.This pulse of current causes the coding relay 7LCTP to pick up duringthe on code period of the reverse code and completes at its frontcontact 36 a connection by which a pulse of current is supplied frombattery 7B3 to the rails of the section 7RT, the connection includingthe winding of relay 7RLTR. The battery 7133 is poled to supply currentof the reverse polarity to which the relay 7RLTR does not respond.

A polar bias approach relay '7RAR is connected in muitiple with thewinding of relay YRLTR through a front contact 96 of the coding relay7LCTP and the portion of current that flows in the approach relay 7RARflows in the direction proper to pick up that relay but the magnitude ofthe current is sufficient to pick up this relay only under certaintraffic conditions to be described hereinafter.

This reverse code pulse of current supplied to the section 7RT isreceived at the left-hand end of the section by the connection includingthe bias polar code following relay 7RRTR and back contact 35 of thecoding relay 7RCTP, the current flowing in the relay 7RRTR in adirection proper to pick up that relay. Code operation of the relay7RRTR functions to repeat the reverse code in the track circuit ofsection 7LT through coding relay 7LCTPR, this latter relay beingcontrolled by a circuit that includes terminal B, front contact 32 ofrelay ATR for the section AT, front contact 54 of the code followingrelay 7RRTR, front contact 55 of relay NWR, winding of relay 7LCTPR andterminal N. When the coding relay 7LCTPR picks up closing front contact31, the track battery 7B4 is connected across the rails of section 7LTthrough the winding of polar bias code following relay 7LTR. The battery734 is poled for the reverse code pulse supplied to the track circuitsection of 7LT to be of the reverse polarity to which the relay 7LTRdoes not respond. This reverse code pulse of current is received at theleft-hand end of the section 7LT and flows in the connection includingcode following relay 7 RTR and back contact 28 of the code transmittingrelay 7RCTPR and the current flows in the code following relay 7RTR inthe direction to pick up the relay. When relay 7RTR is picked up closingits front contact 81 a simple circuit is completed for a slow releasedetector relay 7RTFPR which is retained picked up as long as the relay7RTR is operated at code. When the relay 7RTR picks up closing frontcontact 21 a connection is completed by which current flows from battery5.82 to the rails of the section 6RT through the winding of the codefollowing relay 6LTR. The battery 632 is poled to supply current of thereverse polarity to which the relay GLTR does not respond.

This reverse code pulse supplied to the right-hand end of the section6RT is received at the left-hand end of the section and flows throughthe connection including relay 6TR and back contact 19 of the codingrelay 6RCTPR and the current flows in the winding of relay 6TR in thedirection proper to pick up the relay. During the on code period of thiscode pulse when the relay 6TR is picked up closing front contact 65, asimple circuit is completed for energizing a repeater relay 6RTP andthat relay is operated to follow the code operation of the relay 6TR.

Operation of the repeater relay 6RTP closing its front contact 56completes a simple circuit by which current is supplied to a slowrelease block detector relay 6RTFPR, the connection being completedthrough front contact 57 of the track relay 5TR as will be apparent byan inspection of the drawing. Thus, the block detector relay 6RTFPR isretained picked up as long as reverse code pulses are received atlocation 6 to indicate that the block is clear. Furthermore, operationof the relay 6RTP to operate its contact member 72 causes pulses ofcurrent to be supplied to a decoding transformer 6D to which twodecoding units DU and lZfiDU are connected but these units are tuned torespond to code operation of the relay 6RTP at the 75 and code rates,respectively, and are nonresponsive to the low code rate of the reversecode.

It follows from the above description of the normal track circuits forthe block between the passing sidings 6P3 and SP8, that the coder 3tlCTand its associated repeater function to supply a normal pulse of a lowcode rate of 30 pulses per minute to the left-hand end of the block.This normal code pulse which has an on code period of approximatelyone-half second out of every two seconds is of normal polarity and iscascaded from the location 6 at the left-hand end of the block to thelocation 8 at the right-hand end of the block to operate a blockdetector relay 8LTFPR and also to cause the decoding transformer 3D andan associated coding relay 3LCTPR to generate a reverse code which hadan on code period of approximately one-half second. This reverse codepulse is of reverse polarity and is cascaded through the track circuitsback to the location 6 where it is effective to energize block detectorrelay eRTFPR, these two detector relays being retained energized as longas pulses of current of the 30 code rate are supplied to the trackcircuits. Also, the two slow release relays 6LTFPR and 7RTFPR at thelocation of signal 7L and the two slow release relays 7LTFPR and SRTFPRat the location of signal 7R are retained energized in response to thecoded track circuit current, these relays at these two signal locationsserving as directional detector relays as will appear shortly. Also, atthe location of the spur track AS the normal and reverse codes arerepeated as long as the detector track section AT is unoccupied and thecorresponding track relay ATR is picked up.

At the location 6, the block detector relay 6RTFPR when energizedclosing front contact 58, completes a connection by which power frompositive terminal B is connected to a terminal 59 of a field station 6of an associated centralized traflic control system. In response toenergy on terminal 59, an indication is sent from the field station 6 toa central oflice of such centralized traffic control system causingpositive energy to appear at a terminal 69, the relationship beingindicated by a dotted line. An indication relay 6RHK is connected toterminal 69 and is energized to indicate at the oflice the cascading ofthe reverse code pulses through the track circuits of this stretch ofrailway track. Similarly, at location 8 the block detector relay 8LTFPRwhen picked up closing front contact 61 completes a connection by whichpositive energy is placed on a terminal 62 of field station 8 of thecentralized traffic control system and as indicated by a dotted line, anindication is sent'from that field station through the centralizedtraffic control system to the office causing energy to be applied to aterminal 63 of the office and to which terminal an indication relay SLHKis connected with the result this relay is energized and provides anindication that the normal code pulses are cascaded through the trackcircuits of the stretch of track. In accordance with the usual custom insuch centralized traffic control system, these 9 twov relays 6RHK andSLHK are used to control an indication light to be referred to shortly,for indicating the condition of the block between the locationsv 6 and8.

The centralized trafiic control system is shown only conventionallybecause it can be any one of several known arrangements and the specifictype of system used forms no part of my present invention. 7

I shall next describe the operation of the apparatus to clear a signalto permit a train to move through the block. I shall further considerthat the signal 6RA at the left-hand end of the block is to be clearedto permit a train to move from left to right through the block.

To clear the signal 6RA to enable the train to move into the block, acontrol from the oflice of the centralized tratfic control system issent to field station 6 through the centralized traffic control system,this control being initiated by placing a lever 6R at the otfice to theright as viewed in the drawing so that positive energy is applied to aterminal 12. As indicated by a dotted line, energy applied to terminal12 serves to cause a corresponding control voltage to appear at aterminal 11 of the field station 6 of the centralized traffic controlsystem. With positive energy made to appear at terminal 11, the relay6RT-18R is energized by a pickup circuit that includes terminal 11 andfront contact 64 of the track relay STR. Then, with the relay 6RHSRpicked up, it is retained energized by a stick circuit that includes aterminal 9 of the centralized traffic control system, its own frontcontact 66, and front contact 64 of track relay STR.

With the relay 6RHSR picked up in response to this remote control, theopening of back contact 15 of the relay opens the circuit by which thecoding relay GRCTPR is energized and this relay ceases to be operated atthe 30 code rate and the supply of normal code pulses to the trackcircuit of the section 6RT is discontinued. This discontinuing of thesupply of the normal code pulses to the track circuit of section 6RT isreflected through the block and at location 8 the block detector relaySLTFPR is deenergized and released at the end of its slow releaseperiod. The opening of front contact 45 of the detector relay 8LTFPRinterrupts the circuit by which the induced voltages from winding 43 ofthe decoding transformer 8D are supplied to the coding relay SLTCPR andthat relay ceases to be operated with the result the reverse code pulsesare no longer supplied to the track circuit for the section 8LT. Thisdiscontinuing of the reverse code pulse is reflected through the blockand at location 6 the detector relay 6RTFPR is no longer energized andis released at the end of its slow release period. It follows that thedirectional relays GLTFPR and 7RTFPR at the location of signal 7L andthe directional relays 7LTFPR and SRTFPR at the location of sign 7R arealso deenergized and released when the supply of 39 code track circuitcurrent is cut off.

At location 8 the deenergizing of the relay SLTFPR due to thediscontinuing of the normal 30 code track circuit current causes acircuit to be completed by which coded track circuit current of the 75or 120 code rate is supplied to the track circuit of section 8LT. Atlocation 8 two code transmitters or coders 75CT and 120CT are providedand are constantly operated. Also, there.

is provided a trafiic controlled relay 8RAHR which is associated withthe signal 8RA and controlled by traffic conditions to the right oflocation 8 by means not shown but which would be in accordance withstandard practice. This relay 8RAHR governs a circuit by which eitherthe coder 75CT or 120CT is selected for operating the code transmittingrelay 8LCTPR. Assuming that the relay tiRAHR is released and the signal8RA is at stop, the back contact 76 of relay SRAHR completes a circuitextending from terminal B through contact 75 of the coder 75CT, backcontact 76 of relay SRAHR, back contact 77 of a signal control relay8LFSR for signal fiRA, back contact 45 of detector relay 10 SLTFPR,front contact 46- of approach locking relay SLAS at location 8, frontcontact 47 of track relay QTR and winding of coding relay SLCTPR toterminal N. Hence the coding relay SLCTPR is now operated at the coderate and current pulses from battery 882 to the track circuit of section8LT are supplied at the 75 code. rate due to the operation of contact 39of the code transmitting relay. The polarity of the code pulses suppliedto the track circuit of section 8LT is of the reverse polarity the sameas the reverse code pulses of the 30 code rate. The track circuitcurrent of the 75 code rate is illustrated by the third graph from thetop in Fig. 2. It follows that the code following relay SLTR does notrespond but the code following relay SRTR at the left-hand end of thesection 8LT is operated at this 75 code rate. This code operation ofrelay SRTR causes the directional relay SRTFPR to be energized each timethe front contact 43 of the relay SRTR is closed so that the directionalrelay is picked up and retained energized due to its slow releasecharacteristic. Also, operation of the relay 8RTR at the 75 code ratecauses the relay SRH connected to the 75DU decoding unit to be pickedup. Since the directional relay 7LTFPR is now released a circuit isformed for signal control relay 7RHR, the circuit extending fromterminal B through back contact 48 of code following relay SRTR, frontcontact 78 of relay SRTFPR, back contact 30 of relay 7LTPPR, winding ofrelay 7RHR, and front contact of relay 8RH to terminal N. The relay 7RHRis provided with slow release characteristics and it is picked up andremains picked up closing its front contacts in response to the trackcircuit current of the 75 code rate supplied to the section 8LT. Theclosing of front. contact 49 of the relay 7RHR prepares a circuit foroperating the signal 7R this circuit to be described hereinafter. Theclosing of front contact 86 of the relay 7RHR completes a simple circuitby which a IZtBCT code transmitter provided at the location of signal 7Ris energized and. it operates its contact member 73 at the code rate.The coding relay 7LCTP is now provided with a circuit completed throughfront contact '73 of the 120CT coder and front contact 53 of the signalcontrol relay 7RHR. With the coding relay 7LCTP operated at the 120 coderate, current impulses of the corresponding code rate are supplied fromthe battery 7B3 to the track circuit of the section 7RT through theconnection which includes the winding of the code following relay 7RLTR.These current pulses are of the reverse polarity the same as the reversecurrent pulses of the 30 code rate supplied from battery 733. The codepulses of the 120 code rate are illustrated by the graph fourth from thetop of Fig. 2. It follows that relay 7RLTR is not picked up by thistrack circuit current of the 120 code rate supplied from battery 7133,but that these track circuit current pulses of the 120 code rate thussupplied to the section 7RT flow in the connection including the codefollowing relay 7RRTR in a direction to operate that relay and thatrelay is operated at the 120 code to repeat the code in the trackcircuit for the section 7LT. That is, operation of the relay 'iRRTRcauses the coding relay 7LCTPR to be operated at a corresponding codedue to the circuit previously traced and which includes the frontcontact 32' of track relay ATR and front contact 54 of relay 7RRTR.Operation of the relay 7LCTPR at the 120 code rate causes acorresponding code rate of current pulses to be supplied from battery7B4 to the track circuit for section 7LT, the connection including thewinding of code following relay 7LTR but which relay does not respond tothe polarity of these code pulses because the battery 784 is poled tosupply current of the reverse polarity. At the left-hand end of thesection 7LT, the coded current supplied to that section causes the relay7RTR to be operated at the 120 code andv in turn this relay causes codepulses of the 120 code rate and reverse polarity to be supplied fromtrack battery 6B2 to the track circuit for the section 6RT, theconnection including front contact 21 of relay 7RTR and the winding ofthe relay GLTR, which latter relay is nonresponsive to current of thispolarity. Furthermore, the closing of front contact 81 of relay 7RTRcompletes the circuit for detector relay 7RTFPR and that relay is pickedup. The coded current thus supplied to the track circuit of section 6RTcauses the relay 6TR to be operated at the corresponding code and inturn the repeater relay 6RT? is correspondingly operated. Operation ofthe repeater relay 6RT? to operate its contact member '72 suppliescurrent pulses to the decoding transformer 6D at a corresponding codeand the coding unit 120DU is effectively energized to cause the relay6RDR connected with that decoding unit to be energized and picked up.The picking up of the relay GRDR closing front contact 82 completes acircuit by which a signal control relay 6RAHR is energized and whichcircuit can be traced from terminal B through front contact 89 of trackrelay STR, back contact 84 of relay 6RFSR, front contact 82 of relay6RDR, winding of relay 6RAHR, and to terminal N through a contact 83operated by the track switch of siding 6P8 and closed when the switch isset for the main track.

When the relay 6RAHR is picked up closing front contact 85 and the relay6RDR is energized closing front contact 86, a circuit is completed forthe green light G of the signal 6RA and that lamp is illuminated tocause the signal 6RA to display a clear signal indication, the circuitbeing completed from terminal B through front contact 148 of an approachcontrol relay 6RAPR, front contact 85 of relay, 6RAHR, front contact 86of relay 6RDR and lamp G of signal 6RA to terminal N. The relay 6RAPR isassociated with signals 6RA and 6RB and it is approach controlled in anyof the well known arrangements not shown.

In the event a train is to move from the passing siding 6PS instead offrom the main track into the block and the track switch is reversed sothat the contact 83 occupies the position indicated by dotted lines, therelay 6RBH is selected in response to the energizing of the relay 6RDRand with relay 6RBH picked up closing its front contact 154, a circuitis completed through the front contact 155 of relay 6RDR, for the greenlight G of signal 6RB and that light is illuminated for the signal 6R3to display a clear indication for the train on the siding to move intothe block.

Assuming, however, that the signal 6RA has been cleared for a train tomove into the block and the train accepts the signal to enter thesection ST. The shunting of the track relay STR opens the front contact89 of relay STR over which the signal relay GRAHR is controlled and thusthe signal 6RA is set to display its stop indication. Subsequent to therelease of the track relay STR and prior to the release of the signalcontrol relay GRAHR, a pickup circuit is formed for the stick relayGRFSR and that relay is picked up, the circuit including back contact 90of the relay STR and front contact 91 of the relay 6RAI-IR. The relay6RFSR is then retained energized by a stick circuit including its ownfront contact 93 and a back contact 94 of the track relay STR. Therelease of track relay TR opening its front contact 64 opens the circuitfor the remote controlled relay 6RHSR and that relay is released.Furthermore, the opening of front contacts 17 and 57 of the track relaySTR prevents operation of the code transmitting relay 6RCTPR, and theenergization of block detector relay 6RTFPR.

When the train enters the section 6RT and the code following relay 6TRis shunted and it ceases operation with the result that the repeaterrelay 6RTP ceases to operate and the relay 6RDR is deenergized.

When the train clears the track section 5T and the track relay STR isreenergized and picked up, the stick relay 6RFSR is retained energizedby a stick circuit which is now completed over a back contact 95 of theblock detector relay 6RTFPR. This results in the coding relay 6RCTPRbeing operated at the low code rate due to a circuit extending fromterminal B through front contact 10 of coder 30CT, front contact 14 ofrepeater 30CTP, back contact 15 of relay GRHSR, front contact 117 ofstick relay 6RFSR, back contact 16 of relay 6RTFPR, front contact 17 ofrelay STR, front contact 18 of relay 6RAS and winding of the codingrelay 6RCTPR to terminal N. This means that normal pulses of the 30 coderate are supplied to the rails of section 6RT behind the train.

When a train advances to the right and enters the section 7LT, the codefollowing relay 7RTR is shunted and ceases to operate so that the codepulses of the code rate supplied to the section 6RT are discontinued.Also, the slow release directional relay 7RTFPR is deenergized andreleases at the end of its slow release period. This makes no change inthe condition of the apparatus at location 6 but the relay 6LTR at theright-hand end of the section 6RT is operated at the low code rate dueto the code pulses supplied to the track circuit at the lefthand end ofthe section 6RT when the train vacates the section 6RT. With the relay6LTR operated at the low code rate, the code transmitting relay 7RCTPRis operated at the corresponding code rate due to the circuit previouslytraced. Thus there are supplied to the rails of the section 7LT behindthe train normal current pulses of the low code rate from the battery7B1. Also the detector relay GLTFPR is picked up in response tooperation of relay 6LTR.

When the train enters the section AT at the track AS to shunt trackrelay ATR, that relay is released to open its front contact 32 anddisconnect the circuit by which the coding relay 7LCTPR is beingoperated so that code pulses of the 120 code rate supplied to theright-hand end of section 7LT are discontinued. With the train advancingto vacate the section 7LT the code following relay 7LTR is operated inresponse to the normal code pulses supplied to the left-hand end ofsection 7LT and that relay is operated at the low code rate. Thisoperation of the code following relay 7LTR causes the coding relay 7RCTPto be correspondingly operated and code pulses from battery 7B2 aresupplied to the rails ofthe section 7RT when the train advances to theright to vacate the section AT. Thus normal code pulses of the low 30code rate are supplied to the section 7RT behind the train.

As soon as the head end of the train advances into the section 7RT, thetrack relay 7RRTR is shunted and the circuit for the coding relay 7LCTPRis opened at front contact 54 of relay 7RRTR and the repeating of the120 code in the section 7LT continues to be discontinued. It followsthat there is no change in the condition of the apparatus at location 6as the train passes into the section 7RT.

As the train moves to the right and advances to a given distance, say, adistance of the order of 4000 feet from signal 7R, the magnitude of thecode pulses supplied to the section 7RT due to the operation of the codetransmitting relay 7LCTP is increased because of the shunting of theimpedance of the track rails by the train and the portion of the energysupplied to the winding of the approach relay 7RAR becomes large enoughto effectively energize this relay and it begins to be operated at the120 code rate. This operation of the approach relay 7RAR to recurrentlyclose its front contact 97 permits energy to be supplied to the slowrelease relay 7RAPR and that relay is picked up and retained picked upfrom one code pulse to the next. With the relay 7RAPR picked up closingfront contact 98, the circuit is completed for illuminating the lamps ofthe signal 7R, this circuit extending from terminal B through frontcontact 98 of relay 7RAPR, front contact 49 of relay 7RHR, back contact99 of relay 8RDR, the yellow light Y of sig nal 7R and terminal N.Consequently, the signal 7R is 13 now illuminated to display an approachindication to the train moving to the right.

When this train has passed signal 7R into the section 8LT, the codefollowing relay 8RTR is shunted and it ceases to be operated by the codepulses supplied to the section 8LT at location 8. The relay 8RTR notfollowing code, the detector relay SRTFPR is deenergized and releases toopen its front contact 78 at the end of its slow release period. Thiscauses the relay 7RHR to be released and the circuit for the red lamp Rfor the signal 7R is completed at back contact 49 of relay 7RHR, andthat lamp is illuminated causing signal 7R to display a stop and proceedindication.

When the relay 7RHR was first energized to operate the signal 7R, theclosing of front contact 136 of relay 7RHR completed a pickup circuitfor stick relay 7RSR and this stick relay 7RSR is retained energized andin the picked up position subsequent to the train entering the section8LT to deenergize the relay 7RHR due to a stick circuit being completedthrough its own front contact 100 and three multiple paths includingback contact 101 of relay SRTFPR, back contact 102 of relay 7LTFPR andback contact 51 of the code following relay SRTR. It follows that thestick relay 7RSR remains picked up while the train occupies the section8LT.

With the train vacating the section 7RT the code following relay 7RLTRis operated at the normal low code rate due to the code pulses suppliedto the section 7RT at the left-hand end and this operation of the codefollowing relay 7RLTR causes current pulses of the 30 code rate to besupplied to the section 8LT behind the train.

The energizing of the stick relay 7RSR in the manner previouslyexplained completes a circuit for operating the code transmitter orcoder 7CT at signal 7R, this circuit including terminal B, back contact80 of relay 7RHR, front contact 137 of relay 7RSR, the winding of thecoder and terminal N. Also, a circuit is completed for the relay 7LCTPfor operating this relay at the 75 code rate, the circuit beingcompleted from terminal B through contact 106 of coder 75CT, frontcontact 52 of stick relay 7RSR, back contact 53 of relay 7RHR andwinding of relay 7LCTP to terminal N. Hence at this time code pulses ofthe 75 code rate are supplied from battery 7B3 to the right-hand end ofthe track section 7RT. The code pulses of the 75 code rate thus suppliedto section 7RT are of the reverse polarity and are cascaded past thesection AT in the manner previously explained for the reverse codepulses of the 30 code rate and also at the location of signal 7L the 75code pulses are repeated into the section 6RT in the same manner as thereverse low code pulses were cascaded past this point. At location 6,the 75 code pulses received from the track circuit of section 6RT causethe relay 6TR to be operated at the corresponding code rate. Prior tothis time, the coding relay GRCTPR has been operating at the 30 coderate for feeding track circuit pulses from the left to the right. Thusin the sections 6RT, 7LT and 7RT current of the 30 code is beingsupplied from left to right and current of the 75 code is being suppliedfrom right to left. This action results in a scramble of the two codesand causes the track circuit current to have a graph similar to thatillustrated by the fifth graph from the top of Fig. 2. Due to thisscramble of the track circuit current, the detector relay 7LTFPR atsignal location 7R and the block detector relay 6RTFPR at location 6become energized and are retained picked up due to their slow releasecharacteristics. The relay GRTFPR on picking up to open its back contact95 opens the stick circuit for the stick relay 6RFSR and that relay isreleased causing the circuit for the coding relay RCTPR to betransferred from the path through back contact 16 of relay 6RTFTR andfront contact 117 of relay 6RFSR to the path including back contact ofrelay 6RHSR and front contact 16 of relay 6RTFPR, and the coding relay6RCTPR continues to be operated at the code rate. This condition for thetrack circuit between location 6 and the location of signal 7R continuesas long as the train remains in the section 8LT.

Under this condition described above, it is possible to operate signalGRA or 6R8 to permit a following train to enter the block. This is doneby a control being sent through the centralized traffic control systemto apply 3 to terminal 11 and cause relay 6RHSR to be picked 1e mannerexplained for the first train. With the relay fiRHSR picked up openingback contact 15, the circuit for operating the coding relay 6RCTPR isopened and the supply of code pulses of the 30 code rate to the trackcircuit of section 631 is discontinued. Since the relay RHR connected tothe decoding unit DU at location 6 is energized in response to thecurrent of the 75 code rate received from the track circuit of section6RT, the relay oRAHR or 6RBHR is energized over front contact 15%; ofrelay dRHR according to the position of the track switch, and the signal6RA or 6R8 is controlled to display a yellow light for an approachsignal indication to permit a train to enter the block. This circuit forthe signal 6RA, for example, can be traced from terminal B over frontcontact 143 of approach relay GRAPR, front contact of relay dRAHR, backcontact 86 of relay 6RDR and the lamp Y of signal 6RA to terminal N.

Assuming, however, that with the first train in the section 8LT nofollowing move is made, then no change takes place in the track circuitsuntil the train in section 8LT has proceeded out of the block. As thehead end of the train in the section 8LT advances to a distance of theorder of 4000 feet of signal SRA, the magnitude of the track circuitpulses increases to a value Where the energy supplied to the approachrelay SRAR is effective to energize that relay and cause it to beoperated in step with the code pulses with the result that a slowrelease relay 8RAPR which is controlled over front contact 138 of relay8RAR is picked up closing its front contact 139. The closing of frontcontact 139 of approach relay SRAPR completes a circuit not shown forthe lamps of the signals $RA and 8RB in the same manner that theapproach relay '7RAPR controls the lamp circuits for the signal 7R. Thelamp circuits for the signals 811A and BRB are not illustrated in thedrawing in order to simplify the drawing, these circuits being inaccordance with standard practice and forming no part of the presentinvention. It will be recalled that at the outset it was assumed thatthe signals 8RA and SRB normally display a stop indication, thus the redlamp of these circuits would be illuminated as the train approached thelocation 8. I shall further assume that a control is sent through thecentralized traflic control system and that traffic conditions to theright of location 8 are such that the signal 8RA is operated to eitherits approach or clear indication to permit the train moving from left toright to advance to the right past the signal. When the train advancespast the signal 8RA to enter the section 9T and shunt the track relay9TR, the shunting of that relay will cause the signal to be operated toits stop position in the usual manner.

it will be recalled that While the train occupied section 8LT codepulses of the 30 code rate were supplied to the track circuits fromlocation 6 and impulses of the 75 code rate Were supplied from locationof signal 7R through the track circuits to the location 6.

After the rear end of the train has vacated section 8LT and it stilloccupies section 9T, the circuit by which the coding relay 8LCTPR isoperated by the 750T coder is opened at front contact 47 of relay 9TR.Since normal code pulses of 30 code are supplied to section 8LT the codefollowing relay 8LTR and its repeater SLTPR are operated and the blockdetector relay 8LTFPR is picked up.

When the train vacates the section 9T and relay 9TR is reenergizeo andcloses its front contact 47, the coding relay 8LC IP responds to thereverse code voltages induced in winding 43 due to operation of relaySLTPR in response to the normal pulses of the code. At the signallocation 7R both directional relays SRTFPR and 7LTFPR are now energizedso that the stick relay 7RSR is deenergized to open front contact 52over which the code pulses are being supplied to coding relay 7LCTP andin turn to the track circuits to the left of signal 7R and the reverse30 code pulses again generated at location 8 are cascaded through thetrack circuits. It follows from the foregoing that quickly after thetrain vacates the block the track circuits are restored to their normalcondition under which the low code rate of 30 pulses per minute is used.

During the entire time the train moving from left to right was in theblock, either the block detector relay SLTFPR at location 8 or the blockdetector relay 6RTFPR at location 6 or both were released causing one ofthe indicating relays SLHK or 6RHK or both at the oflice of thecentralized tratfic control system to be deenergized so that the circuitfor the block indication light 6--8 is completed and the lampilluminated, indicating that there is a train in the block. This lampcircuit can be traced from terminal B over back contact 107 of ofiicerelay 8LDK, back contact 108 of ofiice relay 6RDK, either back contact149 of relay iiLHK or back contact 150 of relay 6RHK and lamp 6-8 toterminal N. When the signal 6RA is cleared to permit the train to moveinto the block, the relay 6RDR at location 6 is picked up closing frontcontact 151, and energy is applied to terminal 152 of the field stationat location 6 and the centralized trafiic control system so operatesthat energy is applied to terminal 153 at the office and relay 6RDK atthe office is energized and picked up opening back contact 108 andcausing the block lamp 6-8 to remain dark because otherwise the circuitfor the lamp'68 would be completed due to the block detector relaySLTFPR being released at this time and the office indicator relay SLHKdeenergized. That is, the block light 68 is dark until the train movespast signal 6RA and enters the block.

The operation of the apparatus for controlling signal 8LA or 8LB topermit a train to enter the block from the right is substantially thesame as the operation by which the signal 6RA is controlled to permit atrain to move into the block from the left and the operation of theapparatus for clearing the signal SLA or SLB need be described only asrequired to point out the manner in which the operation differs from theoperation when the signal 6RA is controlled. When the lever 8L at theoflice of the centralized traffic control system is moved to the left tocomplete a circuit by which energy is supplied to terminal 1119 of thecentralized traffic control equipment and positive energy is made toappear at terminal 111 of the field station 3, the remote controlledrelay 8LHSR is energized and picked up providing the track relay 9TR ispicked up closing its front contact 112. The relay SLHSR is providedwith a stick circuit by which it is supplied with energy from terminal113 through its own front contact 114, and front contact 112 of trackrelay 9TB. after the control from the office is completed. The relaySLHSR on picking up opening back contact 44 opens the circuit by whichcode pulses of the 30 code rate generated at winding 43 are supplied tocoding relay 8LCTPR and the reverse low code pulses are no longersupplied to the track circuit of section 8LT. This causes the blockdetector relay 6RTFPR at location 6 to become deenergized and released.This in turn causes the normal pulses of the 30 code rate to bediscontinued due to the opening of the circuit for the coding relay6RCTPR at front contact 16 of relay 6RTFPR. The coding relay 6RCTPR isnow operated at the 75 code rate due to a circuit which can he tracedfrom terminal B through contact 115 of coder 75CT at location 6, backcontact 116 of a relay -6LAHR, which is governed by trafiic conditionsto the left of location 6 in the usual manner by circuits not shown tocontrol the signals 6LA and 6LB, and which signals are assumed as beingset at stop, back contact 117 of stick relay 6RFSR, back contact 16 ofdetector relay 6RTFPR, front contact 17 of track relay STR, ront contact18 of approach locking relay 6RAS and winding of the coding relay 6RCTPRto terminal N. With track circuit current of the 75 code rate and ofnormal polarity supplied from battery 631 to the track circuit ofsection 611T due to operation of relay 6RCTPR, the relay LTR at theright-hand end of section 6RT is operated at the 75 code rate and thedetector relay 6RTFPR is picked up. Also, the relay 7LH at location ofsignal 7L is energized and picked up through the decoding unit 75DUprovided at this location due to operation of relay 6LT R. Hence thesignal control relay 7LHR is energized and picked up by a circuitextending from terminal B through back contact 22 of relay 6LTR, frontcontact 118 of relay 6LTFPR, back contact 119 of relay '7RTFPR, windingof relay 7LHR and front contact 120 of relay 7LH to terminal N.

The relay 7LHR is slow releasing and remains picked up closing frontcontact 51 to prepare a circuit by which the G or Y lamp of signal 7Lcan be approach controlled in a manner tobe pointed out shortly. Therelay 7LHR on picking up closing front contact 121 completes a simplecircuit for operating a coder 1120GT at the location of signal 7L andthis coder then operates its contact 122 at the 120 code rate causingthe coding relay 7RCTPR to we energized and operated at thecorresponding code rate due to a circuit completed through front contact27 of relay 7LHR. This results in code pulses of 120 code rate beingsupplied from battery 7B1 to the section 7LT, the pulses being of normalpolarity. Code pulses of the 120 code rate are cascaded through thetrack circuits to the location 8 in the same manner explained forcurrent pulses of the 30 code. The code following relay 8LTR and itsrepeater relay 8LTPR at location S are operated at the corresponding l20code with the result the control relay SLDR is energized through thedecoding unit IZtiDU. This in turn selects the circuit for signalcontrol relay 8LAHR, the circuit including te.- minal B, front contact69 of track relay 9TR, contact 29 of relay sLFSR, front contact 124 ofrelay SLDR, winding of relay 8LAHR, contact 125 operated by the switchof passing siding 3P5 and closed when the switch is set at its normalposition and terminal N. With relay LAl-IR selected, then a signalcircuit is completed from terminal B through front contact 126 of anapproach lighting relay SLAPR, front contact 127 of relay SLAHR, frontcontact 128 of relay SLDR, and green lamp G of signal SLA to terminal N,and this signal is displayed to permit the train to enter the block.

The operation of the apparatus for the train as it moves through theblock from right to left up to and beyond the signal 71. is similar tothat described for a train moving through the block from left to rightand this operation of the apparatus will be apparent from an inspectionof the drawings taken in connection with the description of theoperation of the apparatus for the train moving from left to right.

However, when the train moves past signal 8LA through section QT andclears section 9T to permit track relay JTR to be reenergized, the stickrelay 8LFSR is retained picked up by its stick circuit completed at backcontact 1193 of code repeater relay 3LTPR. This results in tr e codingrelay SLCTPR not being operated and no coded current is supplied to therails of section 8LT behind the train. Thus there is provided no codedtrack circuit current behind the train moving to the left until 1Toccupies the section 6R1". When the train moving to the left is entirelywithin the section 6RT, the coding relay 7RCTPR for section 7LT will beoperated at the 75 code rate causing current of the 75 code to besupplied from battery 7B1 to the section 7LT. This is so because :relay7RCTPR is operated at the 75 code rate due to the circuit completed atfront contact 133 of the coder, front contact 26 of stick relay 7LSR,back contact 27 of relay 7LHR and winding of coding relay 7RCTPR toterminal N.

When the first code pulse of the 75 code rate thus supplied to thesection 7LT is cascaded through the track circuits and applied to therelay SLTR at location '8, the relay SLTR and in turn the repeater relay8LTPR are picked up causing the block detector relay SLTFPR vto beenergized and picked up. This results in the stick relay SLFSR beingdeenergized and released due to the opening of its stick circuit. At thefirst off period of the 75 code current supplied at signal 7L and therepeater relay 8LTPR at location 8 is released, the voitage induced inthe secondary winding 43 of the transformer 8D is of the polarity topick up the coding relay SLTCPR and cause a reverse code pulse to besupplied to the section LT from battery 8B2. This reverse code pulsewill be cascaded through the track circuits to the location of signal7L. These two pulses, one fed from signals 7L to the location 8 and theother fed from location 8 to signal 7L will be of the same code rate butthe on time of relay 8LCTPR may be greater than the off time of relay7RCTPR. There will he, however, some overlapping of the pulses, and thedetector relays will be picked up and the circuits will continue tooperate while the train is in the section 6RT.

With the code following relay 7RTR operated, then code pulses will besupplied to the rails of section 6RT behind the train, the pulses beingof the reverse code polarity.

When the train in section 6RT advances to a reasonable distance from thesignal 6LA, the magnitude of the track circuit pulses supplied at theleft-hand end of section 6R1 increases to a value great enough to causethe approach relay 6LAR to be operated and in turn cause the approachcontrol relay ELAPR to be energized, the circuit being completed overfront contact 104 of the approach relay 6LAR. The picking up of relay6LAPR closing front contact 116 completes the lamp circuits for thesignals 6LA and 6LB but which circuits are not shown in order tosimplify the drawings because they would he of standard arrangement andform no part of my present invention. These lamp circuits and thecontrol thereof by relay 6LAPR would be similar to that which theapproach relay 7RAPR governs the circuits or" the lamps of signal 7R.

If it is desired to operate signal 8LA or SLB to permit a following movethrough the block while the train occupies the section 6RT, a control issent through the centralized traific control system to location 8 toenergize the relay 8LHSR and that relay is picked up opening backcontact 44 to open the circuit by which the coding relay SLCTPR isoperated at this time by voltage induced in winding 43 and the supply ofpulses to the section 8LT from battery 8B2 ceases. Since at this timethe relay 8LTR and its repeater relay 8LTPR are operated by the pulsesof the 75 code rate supplied from the location of signal 7L, the relaySLHR is energized and picked up through the decoding unit 75DU. Theclosing of front contact 135 of relay SLHR completes a circuit forsignal control relay SLAHR or SLAPR according to the position of contact125 operated by the track switch. Assuming that the relay SLAHR isselected, a signal circuit isvcompleted from terminal B through frontcontact 126 of relay 8LAPR, front contact 127 of relay 8LAHR, back 18contact 128 of relay SLDR and the lamp Y of the signal 8LA to terminalN, withthe result that the signal 8LA displays an approach indicationfor a train to move into the block while the first train occupies thesection 6RT.

I shall assume, however, that a control for a following move is not sentand also that the signal 6LA or 6LB is operated in the usual manner topermit the train in section 6RT to move to the left out of the block andthrough the section ST. The reverse code pulses being received from theright-hand end of the block will cause the relay 6TR and its repeater6RTP to be operated at this code rate. This operation of the repeater6RTP causes the detector relay 6RTFPR to be picked up closing its frontcontact 16 and to thereby complete the circuit for operating codingrelay 6RCTPR at the 30 code rate and cause a normal pulse of 30 coderate to be supplied to the track circuit for the section 6RT.

This normal code pulse operates the code following relay 6LTR at theright-hand end of section 6RT and causes the detector relay 6LTFPR to bepicked up to open at its back contact 134 the third path of the stickcircuit for relay 7LSR and that relay is released. With relay 7LSRreleased the circuit of the coder 75CT and the circuit for operating thecoding relay 7RCTPR are opened and the supply of code pulses of the 75code rate to the section 7LT is discontinued. The coding relay 7RCTPR isnow operated by its original circuit including front con tact 25 ofrelay 6LTR, and thus is operated at the normal 30 code rate. It followsthat this normal code pulse is cascaded to location 8 where it causesrelay SLTR' to be operated. This operation of relay 8LTR causes acorresponding operation of the repeater relay 8LTPR which in turnoperates to generate a reverse code pulse of the 30 code rate in themanner previously explained. Consequently, the track circuits are nowrestored to their normal condition.

I shall next describe the operation of the apparatus by which the switchfor the track AS is unlocked to permit a train to move into the blockfor the track AS. At the start a trainman first obtains permission fromthe operator over a telephone circuit in the usual order of operation.With permission obtained, the trainman operates the usual padlock latchfor the switch which operates a contact to open that contact and operatea second contact 158 to close that contact. The opening of contact 140which is interposed in a circuit for relay NWR causes this relay to bedeenergized and released, the circuit extending from terminal B overcontact 141 operated by the track switch and closed only when the switchis set at its normal position, front contact 143 of track relay ATR, acontact 142 of the electric lock SL for the switch, padlock contact 140,and winding of relay NWR to terminal N. The release of relay NWR opensits front contacts 34 and 35 and thereby opens the connection by whichthe normal and reverse 30 code pulses are cascaded between sections 7LTand 7RT with the result the detector relay 6RTFPR at location 6 anddetector relay SLTFPR at location 8 are deenergized and released causingthe supply of low rate code pulses to the track circuits to bediscontinued and track circuit pulses of the 75 code rate to be sentfrom each end of the block toward the location of the track AS. Withrelay 7LTR operated at the 75 code rate, a relay LTFPR is energized by aconnection completed from terminal B over front contact 32 of trackrelay ATR, front contact 33 of relay 7LTR, back contact 34 of relay NWRand winding of relay LTFPR to terminal N. Then during the oif codeperiod and relay 7LTR is released, a circuit is formed from terminal Bover front contact 32 of relay ATR, back contact 33 of relay 7LTR, frontcontact 144 of relay LTFPR and Winding of relay LTBPR and this latterrelay is energized and picked up closing its front contact 145.Similarly, the operation of the relay 7RRTR completes a circuit forrepeater relays RTFPR and RTBPR, the circuit for relay RTFPR'includingterminal B, front contact 32 of relay 19 ATR, front contact 54 of relay7RRTR, back contact 55 of relay NWR, and winding of relay RTFPR. Thecircuit for the relay RTBPR includes front contact 32 of relay ATR, backcontact 54 of relay 7RRTR, front contact 146 of relay RTFPR and thewinding of relay RTBPR, With both relays LTBPR and RTBPR picked upclosing their front contacts 145 and 147, respectively, a circuit iscompleted for the electric lock SL and which circuit includes terminalB, front contact 147 of relay RTBPR, front contact 145 of relay LTBPR,front contact 159 of track relay ATR, padlock contact 158 which is nowclosed, winding of the electric lock SL and terminal N. With theelectric lock SL thus energized, the trainman can operate the trackswitch and permit the train to move from track AS onto the main track ofthe block. It is to be noted that if signal 6RA at location 6 or signal8LA at location 8 has been cleared to permit a train to enter the block,then code from only one end of the block would be supplied to the trackcircuits with the result that one or the other of the relays 7LTR or7RRTR at the location of the track AS would not be operated and theelectric lock SL would not be enerized.

g It is to be pointed out that in the event a remote control cannot besent from the oflice of the centralized traflic control system tostation 6 for energizing the relay 6RHSR and thereby control signal 611Aor 6RB to permit a train totenter the block, the relay 6RHSR can beenergized by means of a manually operable key or push button 6PB.permission and momentarily close the key 6PB and thereby complete asimple pickup circuit for relay 6RHSR after which the relay would beretained energized by its stick circuit. up the apparatus functions inthe same manner as previ ously described to clear signal 6RA or 6R8 topermit a train to enter the block, the apparatus restoring to its normalcondition when the train has moved through the block. Similarly, a pushbutton SPB at location 8 enables emergency operation of the apparatus tobe effected to permit a train to move through the block from right toleft.

Although I have herein shown and described but one form of single trackrailroad signal system using coded track circuits embodying myinvention, it is to be understood that various changes and modificationsmay be made therein within the scope of the appended claims withoutdeparting from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In a single track two direction signal system using coded trackcircuit currents of selected code rates of a relatively high order toreflect diiferent traflic conditions, the combination comprising, atrack block formed with a plurality of track sections, a low rate codermeans at a selected end of the block having a code cycle of a relativelylow code frequency as compared with traffic reflecting code frequenciesand provided with contact means which is closed a relatively shortselected portion of each code cycle; track circuit means for each saidsection and including a battery, a polar code following relay and acoding contact at each end of the section; each said coding contactoperable to a first and a second position, each said code followingrelay connected across the rails of its section over the first positionof the associated coding contact and connected in series with theassociated battery across the rails over the second position of theassociated coding contact, the two batteries of each section being poledreverse to each other, the two polar code following relays of eachsection poled to respond only to the polarity of the current supplied bythe battery at the opposite end of the section, circuit means at eachjunction of said sections including contacts of the code followingrelays at that junction for In this emergency a trainman would obtain Itfollows that with relay 6RHSR picked each code following relay tooperate the coding contact of the adjacent section, other circuit meansincluding said low rate coder contact means when closed to control thecoding contact for the end of the track section at said selected end ofthe block and thereby cascade a low rate code pulse through the trackcircuits to the opposite end of the block, said pulse being of normalpolarity, code generating means including a battery and a transformercontrolled by the code following relay at said opposite end of the blockto generate a voltage in the transformer in response to said code pulseof said normal polarity, means including a polar relay connected to saidtransformer responsive to said generated voltage and operable to operatesaid coding contact at said opposite end of the block for a differentselected portion of said code cycle and thereby cascade a low rate codepulse of reverse polarity through the track circuits to said selectedend of the block, and a detector relay controlled by the code followingrelay at said selected end of the block to detect the cascading of saidnormal and reverse code pulse through the track circuits.

2. In a single track two direction signal system using coded trackcircuit currents of selected code rates of a relatively high order toreflect different traflic conditions, the combination comprising, atrack block formed with a plurality of track sections, a low rate codetransmitter at a selected end of the block having a code cycle of arelatively low code frequency as compared with traific reflecting codefrequencies and associated with contact means which is closed arelatively short selected portion of each code cycle; track circuitmeans for each said section and including a battery, a polar codefollowing relay and a coding contact at each end of the section; eachsaid coding contact operable to a first and a second position, each saidcode following relay connected across the rails of its section over thefirst position of the associated coding contact and connected in serieswith the associated battery across the rails over the second position ofthe associated coding contact, the two batteries of each section beingpoled reverse to each other, the two polar code following relays of eachsection poled to respond only to the polarity of the current supplied bythe battery at the opposite end of the section, circuit means at eachjunction of said sections including contacts of the code followingrelays at that junction for each code following relay to operate thecoding contact of the adjacent section, other circuit means includingsaid low rate code transmitter contact means when closed to control thecoding contact for the end of the track section at said selected end ofthe block and thereby cascade a low rate code pulse through the trackcircuits to the opposite end of the block, said pulse being of normalpolarity, code generating means including a battery and a transformercontrolled by the code following relay at said opposite end of the blockto generate a voltage in the transformer in response to said code pulseof said normal polarity, means including a polar relay connected to saidtransformer responsive to said generated voltage and operable to operatesaid coding contact at said opposite end of the block for a differentselected portion of said code cycle and thereby cascade a low rate codepulse of reverse polarity through the track circuits to said selectedend of the block, two detector relays one controlled by the codefollowing relay at each end of the block, and a block indicatorcontrolled by said two detector relays.

3. In a single track two direction signal system using coded trackcircuit currents of selected code rates of a relatively high order toreflect different traffic conditions, the combination comprising, atrack block formed with a plurality of track sections, a low rate codetransmitter at a selected end of the block having a code cycle of arelatively low code frequency as compared with traffic reflecting codefrequencies and associated with contact means closed for a selectedportion of each code cycleequal to approximately one-fourth of the codecycle;

track circuit means for each said section and including a battery, apolar code following relay and a coding contact at each end of thesection; each said coding contact operable to a first and a secondposition, each said code following relay connected across the rails ofits section over the first position of the associated coding contact andconnected in series with the associated battery across the rails overthe second position of the associated coding contact, the two batteriesof each section being poled reverse to each other, the two polar codefollowing relays of each section poled to respond only to the polarityof the current supplied by the battery at the opposite end of thesection, circuit means at each junction of said sectionsincludingcontacts of the code following relays at that junction for eachcode following relay to operate the coding contact of the adjacentsection, other circuit means including said low rate code transmittercontact means when closed to control the coding contact for-the end ofthe track section at said selected end of the block and thereby cascadea low rate code pulse through the track circuits to the opposite end ofthe block, said pulse being of normal polarity, code generating meansincluding a battery and a transformer controlled by the code followingrelay at said opposite end of the block to generate a voltage in thetransformer in response to said code pulse of said normal polarity,means including a polar relay connected to said transformer responsiveto said generated voltage and operable to operate said coding contact atsaid opposite end of the block for a different selected onefourthportion of said code cycle and thereby cascade a low rate code pulse ofreverse polarity through the track circuits to said selected end of theblock, slow release detector relays one controlled by each codefollowing relay, and means controlled by each said detector relay forgoverning the supply of said code pulses to the track circuits.

4. in a single track two direction signal system using coded tracltcircuit currents of selected code rates of a relatively high order toreflect different traffic conditions, the combination comprising, atrack block formed with a plurality of track sections, a low rate codetransmitter at a selected end of the block having a code cycle of arelatively low code frequency as compared with traffic reflecting codefrequencies and associated with contact means which is closed for aselected portion of each code cycle equal to approximately onefourth ofthe code cycle; track circuit means for each said section and includinga battery, a polar code following relay and a coding contact at each endof the section; each said coding contact operable to a first and asecond position, each said code following relay connected across therails of its section over the first position of the associated codingcontact and connected in series with the associated battery across therails over the second position of the associated coding contact, the twobatteries of each section being poled reverse to each other, the twopolar code following relays of each section poled to respond only to thepolarity of the current supplied by the battery at the opposite end ofthe section, circuit means at each junction of said sections includingcontacts of the code-following relays at that junction for each codefollowing relay to control the coding contact of the adjacent section,other circuit means including said low rate code transmitter contactmeans when closed to control the coding contact for the end of the tracksection at said selected end of the block and thereby cascade a low ratecode pulse through the track circuits to the opposite end of the block,said pulse being of normal polarity, code generating means including abattery and a transformer controlled by the code following relay at saidopposite end of the block to generate a voltage in the transformer inresponse to said code pulse of said normal polarity, means including apolar relay connected to said transformer responsive to said generatedvoltage and operable to operate said coding contact. at said oppositeend of the block for a different selected one-fourth portion of saidcode cycle and thereby cascade a low rate ,code pulse of reversepolarity through the track circuits'to said selected end of the block,two slow release detector relays each controlled by the code followingrelay at each end of the block, coder means at each end of the blockoperable to supply code pulses of the 75' or code rate, and circuitmeans at each end of the block including a back contact of the detectorrelay at the same end of the block to connect the said coder means atthe same end of the block to said track circuits.

5. In a single track two direction signal system using coded trackcircuit currents of selected code rates of a relatively high order toreflect different traific conditions, the combination comprising, atrack block formed with a plurality of track sections, a low rate codetransmitter at a selected end of the block having a code cycle of arelatively low code frequency as compared with trailic reflecting codesand associated with contact means which is closed for a selected portionof each code cycle equal to approximately one-fourth of the code cycle;track circuit means for each said section and including a battery, apolar code following relay and a coding contact at each end of thesection; each said coding contact operable to a first and a second position, each said code following relay connected across the rails of itssection over the first position of the as,- sociated coding contact andconnected in series with the associated battery across the rails overthe second position of the associated coding contact, the two batteriesof each section being poled reverse to each other, the two polar codefollowing relays of each section poled to respond only to the polarityof the current supplied by the battery at the opposite end of thesection, circuit means at each junction of said sections includingcontacts of the code following relays at that junction for each codefollowing relay to control the coding contact of the adjacent section,other circuit means including said low rate code transmitter contactmeans when closed to control the coding contact for the end of the tracksection at said selected end of the block and thereby cascade a low ratecode pulse through the track circuits to the opposite end of the blocksaid pulse being of normal polarity, code generating means including abattery and a transformer controlled by the code following relay at saidopposite end of the block to generate a voltage in the transformer inresponse to said code pulse of said normal polarity, means including apolar relay connected to said transformer responsive to said generatedvoltage and operable to operate said coding contact at said opposite endof the block for a different selected one-fourth portion of said codecycle, and thereby cascade a low rate code pulse of reverse polaritythrough the track circuits to said selected end of the block, a remotecontrolled relay at each end of said block, and means at each end of theblock including a contact of the remote controlled relay at the same endof the block operable automatically in accordance with trafiicconditions at the entrance to said selected end of the block to controlthe supply of said low code rate pulses to the track circuits at thecorresponding end of the block.

6. In a single track two direction signal system using coded trackcircuit currents of selected code rates of a relatively high order toreflect different traffic conditions, the combination comprising, atrack block formed with a plurality of track sections, a low rate codetransmitter at a selected end of the block having a code cycle of arelatively low code frequency as compared with trafiic reflecting codesand associated with contact means which is closed for a selected portionof each code cycle equal to approximately one-fourth of the code cycle;track circuit means for each said section and including a battery, apolar code following relay and a coding contact at each end of thesection, each said coding contact operable to a first and a secondposition, each said code following relay connected across the rails of23 its section over the first position of the associated coding contactand connected in series with the associated battery across the railsover the second position of the associated coding contact, the twobatteries of each section being poled reverse to each other, the twopolar code following relays of each section poled to respond only to thepolarity of the current supplied by the battery at the opposite end ofthe section, circuit means at each junction of said sections includingcontacts of the code following relays at that junction for each codefollowing relay to control the coding contact of the ad jacent section,other circuit means including said low rate code transmitter contactmeans when closed to control the coding contact for the end of the tracksection at said selected end of the block and thereby cascade a low ratecode pulse through the track circuits to the opposite end of the blocksaid pulse being of normal polarity, code generating means including abattery and a transformer controlled by the code following relay at saidopposite end of the block to generate a voltage in the transformer inresponse to said code pulse of said normal polarity, means including apolar relay connected to said transformer responsive to said generatedvoltage and operable to operate said coding contact at said opposite endof the block for a different selected one-fourth portion of said codecycle, and thereby cascade a low rate code pulse of reverse polaritythrough the track circuits to said selected end of the block, a remotecontrolled relay at each end of said block, means at each end of theblock including a contact of the remote controlled relay at the same endof the block operable automatically in accordance with trafficconditions at the entrance to said selected end of the block to controlthe supply of said low code rate pulses to the track circuits at thecorresponding end of the block, an emergency manually operable key ateach of the blocks, and means at each end of the block including thecorresponding key for selective operation to supercede automatic controlof the corresponding remote controlled relay.

7. In a single track two direction signal system using coded trackcircuit currents of selected code rates of a relatively high order toreflect different traffic conditions, the combination comprising, atrack block formed with a plurality of track sections, a codetransmitter at a selected end of said block having a code cycle of arelatively low code frequency as compared to traflic refiecting coderates, said code transmitter associated with contact means which isclosed a selected portion of each code cycle equal to approximately lessthan one-half the code cycle, track circuit means connected across therails at each end of said sections; each said track circuit meansincluding a polar code following relay, a track battery and a codingcontact; the two track batteries of each section being poled reverse toeach other, and the two polar relays of each section being poled foreach relay to respond only to the polarity of the current supplied bythe battery at the opposite end of the section, at each junction of saidsections the two polar relays of the track circuit means at the junctionbeing operable for each relay to control the coding contact of theadjacent section, means including said contact means of said codetransmitter at said selected end of the block to operate the codingcontact of the track circuit means at said selected end of the blockwhereby code pulses of said low code rate are cascaded through the tracksections from the selected end to the opposite end of the block, saidcode pulses being of normal polarity; code generating means including abattery, a transformer and a contact of the polar relay of the trackcircuit means at said opposite end of the block to generate a voltage ina winding of said transformer in response to said normal code pulse,means including a polar relay connected to said transformer winding tooperate the coding contact of the track circuit means at said oppositeend of the block in response to said generated voltages whereby codepulses of reverse polarity are cascaded through the track sections tosaid selected end of the block to operate the polar code following relayat said selected end of the block, and a block indicator controlled bysaid polar code following relay at said selected end of the block.

8. In a single track two direction signal system using coded trackcircuit currents of code rates of a relatively high order to reflectdifferent traflic conditions, the combination comprising, a track blockformed with a plurality of track sections; a track circuit for eachtrack section having connections across the rails at each end of thesection and provided with a current source, a code following relay and acoding relay at each end of the section; said track circuits beinginterconnected at each junction of said sections and poled to cascade acurrent pulse of normal polarity from left to right through the sectionsand to cascade a current pulse of reverse polarity from right to leftthrough the sections, a code transmitter at the left-hand end of theblock having a code cycle of a relatively low frequency as compared tothe frequencies of traflic reflecting codes and associated with contactmeans closed a selected portion of each said code cycle equal tosubstantially less than onehalf the code cycle, means including saidcode transmitter contact means connected to the track circuit codingrelay at said left-hand end of the block to thereby cascade low codenormal polarity pulses through the track circuits, code generating meansincluding a current source at said right-hand end of the blockcontrolled by the track circuit code following relay at the right-handend of the block, said code generating means operable to create avoltage pulse in response to each said normal current pulse, saidgenerating means connected to the track circuit coding relay at theright-hand end of the block to thereby cascade low code rate reversepolarity pulses through the track circuits, at slow release detectorrelay controlled by the track circuit code following relay at theleft-hand end of the block, and a block indicator controlled by saiddetector relay.

9. In a single track two direction signal system using coded trackcircuit currents of code rates of a relatively high order to reflectdifferent trafiic conditions, a combination of elements for supplying tothe track a plurality of traffic codes and a code of relatively lowfrequency with respect to the trafiic codes, the combination comprising,a track block formed with a plurality of track sections; a track circuitfor each track section having connections across the rails at each endof the section and provided with a current source, a code followingrelay and a coding relay at each end of the section; said track circuitsbeing interconnected at each junction of said sections and poled tocascade a current pulse of normal polarity from left to right throughthe sections and to cascade a current pulse of reverse polarity fromright to left through the sections, a code transmitter at the left-handend of the block having a code cycle of a relatively low frequency ascompared to the frequencies of traffic reflecting codes and associatedwith contact means closed a selected portion of each said code cycle,means including said code transmitter contact means connected to thetrack circuit coding relay at said left-hand end of the block to therebycascade low code normal polarity pulses through the track circuits, codegenerating means including a current source at said right-hand end ofthe block controlled by the track circuit code following relay at theright-hand end of the block said code generating means operable tocreate a voltage pulse in response to each said normal current pulse,said generating means connected to the track circuit coding relay at theright-hand end of the block to thereby cascade low code rate reversepolarity pulses through the track circuits, detector relays one at eachend of the block and each detector relay controlled by the track circuitcode following relay at the same end of the block, two remote controlledrelays one at each end of the block, coder means at each end of theblock operable to supply traflic reflecting code pulses, means at eachend of the block including a contact of the remote controlled relay atthe same end of the block to discontinue operation of the correspondingtrack circuit coding relay, and means at each end of the block includinga back contact of the corresponding detector relay to connect the saidcoder means to the track circuit coding relay at that end of the block.

10. In a single track two direction signal system, the combinationcomprising, a track block formed with a plurality of track sections,coding means at a selected end of said block operable to divide timeinto recurrent cycles of a selected time interval and having a contactwhich is operated to one position for a selected portion of each cycle,means including a current source and said contact connected across therails at said selected end of the block to supply a current pulse duringsaid selected portion of each cycle, said current source poled for saidpulse to be of a normal polarity, a first means including a polar relayand a current source at each junction of said track sections havingconnections to the rails of each of the adjoining sections and poled tocascade said normal current pulse from the section toward said selectedend to the section away from said selected end; code generating meansincluding a polar relay, a transformer, a coding relay and a currentsource at the nonselected end of said block; said code generating meansconnected to the rails adjacent said nonselected end and operable tosupply to the rails another current pulse in response to said normalpulse received at said nonselected end, said generating means poled forsaid another pulse to be of reverse polarity, said another pulse beingof a duration equal to a second selected portion of said cycle the sumof said selected portion and said second selected portion being no morethan onehalf of each cycle, a second means including a polar relay and acurrent source at each junction of said sections and having connectionto the rails of the adjoining sections and poled to cascade said anotherpulse from the section away from said selected end of the block to thesection toward the selected end, another polar relay connected to therails adjacent said selected end and poled to respond to current pulsesof reverse polarity,

26 and a slow release block indication relay controlled by said anotherpolar relay.

11. A system for supplying to a track a code of relatively low frequencyduring non-occupancy of the track, the system comprising in combination,a track section, track circuit means for said section and including acoding relay, a bias polar code following relay and a battery for eachend of the section, each said code following relay in series with thebattery for the same end of the section connected across the sectionrails over a front contact of the coding relay for the same end of thesection and connected across the rails exclusive of the batteiy over aback contact of the coding relay, said batteries poled reverse to eachother, each said bias polar code following relay poled to respond tocurrent of the polarity supplied by the battery for the opposite end ofthe section, coder means including a driven code transmitter for aselected end of the section and operable to divide time into recurrentcycles and having a contact means closed for only a selected portion ofeach cycle, circuit means including a current source and said contactmeans connected to the coding relay for said selected end of the sectionto pick up that relay for said selected portion of each said cycle, acode generating means for the nonselected end of the section andincluding a direct current source and a transformer having a primary anda secondary winding, means including a front contact of the codefollowing relay for said nonselected end of the section to connect saiddirect current source to said primary winding to induce voltages in saidsecondary Winding, the induced voltage being of reverse polarity whensaid direct current source is disconnected from said primary Winding,circuit means having connections to said secondary winding and to thecoding relay for said nonselected end of the section, and said codingrelay for said nonselected end being a bias polar relay and poled torespond only to said voltage of reverse polarity.

References Cited in the file of this patent UNITED STATES PATENTS2,350,657 Wynn June 6, 1944 2,357,518 Judge Sept. 5, 1944 2,561,956Staples July 24, 1951

